Laser amplifiers have many possible applications in future fiber optic communication systems, including overcoming losses in transmission and switching systems and in increasing receiver sensitivity. Very low facet reflectivities, which have proven to be difficult to achieve, are the key to producing high quality semiconductor laser amplifiers.
Current research efforts regarding semiconductor laser amplifiers have focused on the reduction of facet reflectivity due to the enhanced performance of certain key amplifier properties. In an ideal travelling-wave amplifier, the incoming light enters one side of the amplifier, undergoes gain as it propagates, and exits without loss from the other side. Unfortunately, the gain available from the amplifier is limited since the amplifier must avoid positive optical feedback necessary for lasing and, more specifically, must prevent the Fabry-Perot resonances exhibited during lasing operation because they cause the gain to be strongly wavelength-dependent. Since the feedback mechanism is influenced by the degree of reflectivity which exists at the device facets, a low facet reflectivity is extremely important for suppressing the Fabry-Perot resonances and achieving high quality amplifier performance. Furthermore, the noise performance of the amplifier improves with decreasing facet reflectivity.
Two conventional approaches have been used to reduce the reflectivity of the semiconductor amplifier facets: coating the facets with an anti-reflection material and tilting the facets. Zah et al. in Electronics Letters, 1987, disclose a double-channel ridge-waveguide travelling-wave amplifier (TWA) having angled facets which suppress Fabry-Perot resonances by reducing the amount of reflected light that is coupled back into the amplifier waveguide. The amplifier achieved a residual modal reflectivity of about 2.times.10.sup.-3 with the angled facet structure, while a further reduction in reflectivity to a value of 10.sup.-4 occurred when a 1% anti-reflection coating was applied to both facet surfaces. The combination of angled facets and anti-reflection coatings also appears in a GaInAsP near-travelling-wave (NTW) laser amplifier disclosed by Zah et al. in Electronics Letters, 1988, that has experimentally demonstrated a lower facet reflectivity of 5.8.times.10.sup.-4 However, this value has been difficult to achieve reproducibly for a number of reasons. One reason concerns the sensitivity of facet coating to errors in thickness and index of refraction, where the best results have been obtained by the expensive process of individually coating each facet using in-situ monitoring of its output. Furthermore, the reflectivity of facet coatings can degrade with time due to environmental effects. In addition, the reflectivity of angled facets can only be made very small with very exact control of active dimensions, again indicating that only a small yield of low reflectivity devices is possible with conventional techniques.
Recently, a new and improved optical amplifier structure capable of achieving reflectivities as low as 3.times.10.sup.-5 was disclosed by Cha et al. in Electronics Letters, 1989. This travelling-wave optical amplifier is distinguished from the aforementioned conventional amplifiers by the presence of a window facet region adjacent to the amplifier facet surface from which the output optical signal appears and is coupled to an optical fiber. An anti-reflection coating is applied to the outer facet of the window at the window-air interface. Disadvantageously, the amplifier suffers from a large coupling loss of 8 dB at the window facet-fiber interface in addition to the above-described fabrication problems associated with the application of anti-reflection coatings.